Hearing loss is generally classified in two types: conductive and sensorineural. Conductive hearing loss is result of a decreasing of the mechanical chain functionality constituted by the external and middle ear, which captures and drives the mechanical energy of the sound to the cochlea. Sensorineural hearing loss is due to the deficiency or damage of the hair cells in the cochlea. Those hair cells are needed to codify the sound into nerve impulses sent to the brain.
While a conductive hearing loss can often be corrected, at least partially by medical or surgical procedures or by using conventional auditory prosthesis that amplifies the sound. To compensate sensorineural hearing loss, many Cochlear Stimulation Systems have been developed, such as the ones described in U.S. Pat. No. 4,400,590 to Michelson, U.S. Pat. No. 5,626,629 A to Faltys et al., U.S. Pat. No. 6,067,474 to Schulman et al., U.S. Pat. No. 6,157,861 A to Faltys et al., U.S. Pat. No. 6,219,580 B1 to Faltys et al., U.S. Pat. No. 6,249,704 B1 to Maltan et al., and U.S. Pat. No. 6,289,247 B1 to Faltys et al. To overcome sensorineural deafness, those systems bypass the hair cells function by means of the direct electrical stimulation of the auditory nerve fibers to create a sound perception in the implanted patient's brain. In most of these systems, electrical stimulation is made through an intracochlear electrode array excited by a suitable external electrical power source.
Previous to the electrical stimulation, each of these Cochlear Stimulation Systems processes the spectrum of frequencies of the input sound. This spectrum is separated into a certain number of frequency bands by using a band-pass filters bank. After this, the amplitude of the output signal of each filter is translated or mapped based on: loudness perceptions criteria of the patient implanted with a Cochlear Stimulation System; and an electrical stimulation current level for each one of the intracochlear electrodes.
Throughout the remainder of this application, including the appended Claims, the term “implanted patient” is used to refer to a patient implanted with a Cochlear Stimulation System.
Some of the algorithms used to translate the input sound into an electrical stimulation current are described in U.S. Pat. No. 3,751,605 to Michelson, U.S. Pat. No. 4,267,410 to Forster et al., U.S. Pat. No. 4,284,856 to Hochmair et al., U.S. Pat. No. 4,400,590 to Michelson, U.S. Pat. No. 4,408,608 to Daly et al., U.S. Pat. No. 4,428,377 to Zollner et al. and U.S. Pat. No. 4,532,930 to Crosby et al. There are operation parameters of the Cochlear Stimulation Systems, which take part in the dynamic range mapping of the input sound into an adequate dynamic range of the electrical stimulation current. The residual cochlear tissue condition and the loudness sensations parameters of the implanted patient are some other parameters taking part too.
Among these parameters are some that refer to the signal processing that the particular Cochlear Stimulation System makes over the input sound and those relative to the electrical stimulation current. There are numerous parameters, such as: the number of channels or frequency bands in which the input sound is separated; microphone signal bandwidth; sound processor sensibility; automatic gain control attack and releasing time; the compression relation of the input sound; the intracochlear electrode associated to each frequency band; and the electrical stimulation current dynamic range for each intracochlear electrode. These parameters, which also include the corresponding stimulation strategy (i.e., SPEAK, ACE, CIS or SAS), are selected to convert the input sound into an electrical stimuli applied to the intra cochlear electrode such as: repetition rate of the biphasic pulse of electrical current; width of the pulse for each channel; elapsed time between positive and negative phases of this pulse (GAP); and the number of maximum spectral peaks periodically chosen for its presentation. The operation of the Cochlear Stimulation System must be adjusted according to the individual needs of the implanted patient to accomplish the greatest benefit as possible. For a particular patient and Cochlear Stimulation System there exist optimal stimulation current set values so that the sound can be intelligible, without being painfully sonorous or as soft as undetectable.
The task for the expert clinician is to find the best set of stimulation parameter values or “MAP” to fulfill the patient needs and that offers the best possible performance of the device. The Cochlear Stimulation System fitting poses some inconvenience if there is not any quantifiable parameter; the construction of this MAP based on the subjective judgment of the implanted subject becomes, in some way, a successive approximation procedure. The determination of this MAP based on some prescribed method that employs a limited number of measures has not proved to be successful in all the implanted patients. Besides, it is not possible to adjust each stimulation parameter value one at the time because most of them interact with each other. For example, the number of functional intracoclear electrodes and the biphasic pulse width of the stimulation electrical current limit the stimulation speed for the intracochlear electrodes.
For these reasons, there are different approaches to this problem in clinical practice. Some professionals use the set values suggested by the manufacturer or some other maps of their own. Others attempt to adjust stimulation parameters on individual electrodes based on psychophysical measures and interpolating these values to the neighbors to adjust the total of the electrodes. The problem still remains the same, nowadays there is no known method for the systematic identification of the optimal MAP for a particular user. Most of the actual methods are difficult to apply on the implanted subject, take too much time, and they are poorly reliable.
Alternative to psychophysical methods are those that use the stapedius reflex (SR), the middle ear response (MER), or the evoked compound action potential (ECAP), as an objective measure of the physiological response to sound. Such methods are disclosed in U.S. Pat. No. 5,626,629 A to Faltys et al., U.S. Pat. No. 6,157,861 A to Faltys et al., U.S. Pat. No. 6,289,247 B1 to Faltys et al., U.S. Pat. No. 6,205,360 B1 to Carter et al., U.S. Pat. No. 6,415,185 B1 to Maltan, U.S. Pat. No. 6,751,505 B1 to Van Den Honert et al., U.S. Pat. No. 6,915,166 to Stecker et al., U.S. Pat. No. 7,043,303 B1 to Overstreet, U.S. Pat. No. 7,076,308 B1 to Overstreet et al. and U.S. Pat. No. 7,107,101 B1 to Faltys.
The example of U.S. Pat. No. 6,157,861 A involves a Cochlear Stimulation System and a fitting method. An electrical stimulation is applied to the intracochlear electrodes, and the middle ear reflex (MER) is measured as a response.
Another example is described in U.S. Pat. No. 6,915,166, where an improved method for intracochlear electrodes selection in a Cochlear Stimulation System is disclosed. While variable intensity electrical stimuli are presented to each intracochlear electrode, evoked compound action potential (ECAP) through an implanted electrode is measured. This and other similar methods offer the possibility to estimate the stimulation electrical current minimum and maximum psychophysical values.
The minimal value or threshold level “T” is the electrical current level which, when applied to the corresponding electrode in a certain frequency band, produces a sound perception sensation in, at least fifty percent (50%) of the cases. The maximum value of this current or comfort level “C” corresponds to a moderately sound perception, without being uncomfortable. Nevertheless threshold and comfort levels might not match with the psychophysical levels when complex sounds or live voice is presented.
It is well known that there is an individual overestimation of the psychophysical levels for intracochlear electrodes when using complex sounds or live voice as a sound stimulus. Overestimating the extremes values of the electrical current level dynamic range reduces the benefit of the Cochlear Stimulation System, especially in noisy environment.
The mentioned tests could be done during or after the implant surgery. However, ideal test conditions are with the implanted patient under sedation. Stimulation with variable intensity current pulses might cause patient annoyance, or even painful sensations, when the patient is awake and alert. The right programming of the Cochlear Stimulation System is particularly difficult; yet most previous adjustments considered to be accurate or suitable have been determined based on feedback expressed by the implanted patient. Suitable adjustments might not occur as the patient feedback is subjective.
Users of the same Cochlear Stimulation System could have different performance in sound and speech perception, due to factors such as non-homogeneous distribution of the residual auditory nerve fibers along the cochlea and the electrical impedance characteristics between auditory nervous fibers and intracochlear electrodes. The patient's restricted communications skills to inform the professional clinician about the quality of his sound perception, contributes to the wide performance variation among users of a same Cochlear Stimulation System, standard free field audiometry is not reliable for children under three years old.
Most objective methods take advantage of the facilities included in a Cochlear Stimulation System, which allow the direct electric stimulation through a pair of intracochlear electrodes and then pick up a signal related to the physiological response of the auditory nerve system. For this purpose, it is necessary that the Cochlear Stimulation System possesses reception and transmission information capabilities to and from external environment for accepting the electrical code stimulation for the intracochlear electrodes and sending back the corresponding response. See the example described in U.S. Pat. No. 5,758,651 issued to Nygard et al. However, Cochlear Stimulation Systems without such facilities, and the lack of hardware compatibility between systems made by different manufacturers, do not allow a generalized use of these tests.
Excluding the role of the speech processor from the system fitting procedure does not assure adequate sound perception in the implanted patient under sound or speech environment. This is because the intracochlear electrodes electrical stimulation current dynamic range, established by means of any of the objective methods known up to now, might not correspond to the dynamic range of the implanted subject sound perception.
It is well known that excitation, by electrical current of different sites along the cochlea, is translated by the brain into a pitch perception that goes from high to low as stimulation moves from base to apex. Nevertheless, a loudness sensation can not be clearly established in the implanted patient because it depends on the local characteristics of the residual cochlear tissue in the vicinity of the stimulation electrode. That is why two contiguous intracochlear electrodes with the same electrical stimulation dynamic range might correspond with an adequate loudness sensation in one case and yet be uncomfortably painful in the other. Once the electrical stimulation dynamic range for the electrodes array has been established, the patient's perception of loudness depends on the residual cochlear tissue properties in the electrode neighborhood and the Speech Processor operation parameters, specifically on the sensitivity and gain. In short, the loudness perception of the implanted patient results from the combined effect of the residual cochlear tissue excitability properties, the stimulation electrical current dynamic range and the speech processor amplifying characteristics. Applicants have determined that the implanted patient's loudness perception is related to the Electrical Cochlear Response and the measurement of some of its parameters.
Accordingly, it is a primary object of the present invention to provide an apparatus and objective method which through the presentation of an external sound stimuli of variable intensity, duration and frequency, and measuring the Electrical Cochlear Response, determine the stimulation electrical current dynamic range for the intracochlear electrode array through a patient's “T” and “C” psychophysical levels estimation; all this without the conscious participation of the implanted patient.
It is a more specific object to provide a unique apparatus and method to evaluate Cochlear Stimulation System performance, to estimate audiometric threshold, and to detect non-functional intracochlear electrodes.
It is another specific object, commensurate with the above-listed objects, to provide an apparatus and methodology suitable for either a pediatric or adult cochlear implant user no matter if the Cochlear Stimulation System possesses bidirectional data transmission facilities to the external environment.